Silver halide has been widely used as a light sensitive component in photographic compositions and elements. Because silver halide is intrinsically sensitive only to blue light, it has often been desirable to impart to silver halide sensitivity to other wavelengths of radiation. This has generally been accomplished through the use of one or more spectral sensitizing dyes such as cyanine dyes. The dye is adsorbed to the silver halide surface and absorbs light or radiation of a certain wavelength. The energy thus absorbed by the dye is transferred to the silver halide to produce a latent image, from which a visible image can be developed during photographic processing. Sensitizing dyes are discussed in James, T. H., ed., The Theory of the Photographic Process, 4th Ed., Macmillan, N.Y., 1977, Chapter 8.
Cyanine dyes that contain a single methine group linking the two basic heterocyclic nuclei typically absorb blue light. The absorption of red light requires that the sensitizing dye contain an extended chromophore such as that of a dicarbocyanine structure, which is characterized by a link of three methine groups.
Oxonol dyes, which are highly susceptible to bleaching by sulfite in the course of photographic processing, have been employed as filter dyes (see, for example, Gaspar, U.S. Pat. No. 2,274,782). When employed for this purpose they are used in high enough concentration to produce an optical density greater than one in the region of predominant absorption. Carroll and Staud, U.S. Pat. No. 2,448,858, disclose that pentamethine oxonol dyes, which are only slightly adsorbed, if at all, on silver halide and act, at best, as weak red-sensitizing dyes, show moderately increased sensitizing action in the presence of basic cyanine dyes. They propose that these cyanine dyes, which in and of themselves confer no appreciable red sensitivity on silver halide emulsions, increase the sensitizing capability of the oxonol dyes. There is no suggestion in this patent of a single dye containing both cyanine and oxonol chromophores.
Ugai, Okazaki, and T. Sugimoto, Kokai Pat. No. SHO64 [1989]-91134, disclose compounds in which a spectral sensitizing dye that can be adsorbed on silver halide is connected by a covalent link to at least one virtually non-adsorbing dye containing at least two sulfo and/or carboxyl groups. These compounds are reported to sensitize both regular silver halide crystals such as cubic and octahedral and irregular crystals such as spherical and flat, and there is no suggestion of their exhibiting morphological selectivity.
The present invention provides photographic elements containing novel red-sensitizing dyes which contain simple cyanine moieties covalently linked to bleachable oxonol chromophores and which are morphologically selective for cubic silver halide crystals.